Semiconductor device package having lead frame structure with integral spring contacts

ABSTRACT

A semiconductor device package which can be readily mounted on a printed circuit board without requiring soldering or intermediate connectors. A supporting substrate has a unique lead frame configuration thereon in which the leads extend around side portions of the substrate and form integral spring contacts projecting from the lower surface of the substrate. The package preferably includes a metallic stud having a head portion mounted in the substrate for receiving a semiconductor device and a rod portion extending from the lower surface of the substrate. The rod portion of the stud can be removably secured in an aperture in a printed circuit board to engage the spring contacts with corresponding conductors on the circuit board.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor devices. More particularly, itrelates to a package for a semiconductor device which is easilymountable on a printed circuit board.

Various schemes have been employed to package semiconductor devices. Oneof the most widely accepted schemes is the dual in-line package (DIP).Briefly, the DIP has a plurality of leads extending from both sides ofan elongated rectangular body. The leads are bent so that they projectdownwardly to form a plurality of linear connector pins bounding thebody of the package parallel to its side portions. The leads aregenerally soldered into corresponding holes in a printed circuit boardin order to make external electrical connection to other devices. Whilethe DIP has proved satisfactory in many applications, it has itsdrawbacks when it is realized that the package may have to be removedfrom the printed circuit board for various reasons. In order to removethe DIP from the printed circuit board, one must unsolder all of theleads. This of course is a time consuming operation which also requiresaccess to a heat source such as a soldering iron. Furthermore, this canresult in permanent damage to the printed circuit board.

In those applications where the package may foreseeably need to beremoved from the printed circuit board, other packaging schemes havebeen suggested. For example, the semiconductor device has been mountedin a leadless-type package. Unlike the DIP, the leadless package doesnot have discrete leads extending from its main body portion.Accordingly, an intermediate connector is required to interface it withthe printed circuit board. One of such connectors, as well as aleadless-type package, is disclosed in U.S. Pat. No. 3,771,109 Bruckneret al. Therefore, while the leadless package permits one to remove itfrom the printed circuit board, for example in order to replace a faultysemiconductor device, it necessarily requires an intermediate connectorin order to make external electrical connection to other devices on theboard. Unfortunately, these intermediate connectors tend to be expensiveand may not provide reliable electrical connection to the conductors onthe printed circuit board. This reduced reliability is due to the factthat multiple interfaces are used to provide the electrically conductivepath between the device and the printed circuit board. Each pad on theleadless package is interfaced with a contact element in the connector,with the contact element, in turn, interfaced with the conductors on thecircuit board. Each interface is a potential source of an unreliableconnection.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is the primary object of this invention to provide asemiconductor device package that can be readily mounted on a printedcircuit board without requiring soldering or intermediate connectors. Itis a further object of this invention to accomplish the primaryobjective while also providing excellent heat dissipation for thesemiconductor device to be packaged. It is a further object of thisinvention to provide a subassembly for the semiconductor device packagewhich provides a continuous interfaceless electrically conductive pathbetween the device and conductors on the printed circuit board.

These and other objects of this invention are accomplished through theuse of a substrate having a unique lead frame configuration. The leadsextend not only on the upper surface of the substrate but extend aroundside portions thereof and form resilient spring contacts which projectfrom the lower surface of the substrate. In such manner the lead frameprovides a continuous interfaceless electrically conductive path forconnecting a semiconductor device mounted on the substrate to conductorson a printed circuit board. Other features of this invention include athermally conductive stud which is mounted in an opening in thesubstrate. The stud includes an end face for receiving a semiconductordevice, and a rod-like body portion extending from the lower surface ofthe substrate. The rod portion of the stud can be removably secured inan aperture in a printed circuit board to engage the spring contactswith corresponding conductors on the circuit board. Preferably, theprinted circuit board includes a metallic heat conductive core whichprovides additional heat dissipation for the semiconductor device. Inone embodiment, the rod portion of the stud is slidably inserted into areceptacle in the printed circuit board aperture. A ferrule of thermallydeformable metal surrounds the receptacle below the circuit board. Theferrule clamps the receptacle securely around the rod portion of thestud to prevent any movement of the package which may deleteriouslyaffect the electrical connections between the spring contacts and theircorresponding circuit board conductors.

BRIEF DESCRIPTION OF THE DRAWING

FIG 1 shows an exploded perspective view of a semiconductor devicepackage made in accordance with one embodiment of this invention;

FIG. 2 shows a sectional view of the assembled package along the lines2--2 of FIG. 1.

FIG. 3 shows an exploded perspective view of another semiconductordevice package utilizing the lead frame structure of this invention; and

FIG. 4 shows a sectional view of the assembled package shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a substrate 10 includes a centrallylocated rectangular recess 12. Recess 12 includes a flat bottom surface14 forming a ledge which is generally parallel with the upper majorsurface 18 of the substrate 10. Recess 12 also includes a generallyrectangular opening 16 extending from surface 14. In this embodiment,recess 12 is approximately 0.25 inch square, whereas opening 16 isapproximately 0.15 inch square. Preferably, substrate 10 is made fromcopper alloy 638. Substrate 10 has two substantially parallel oppositemajor surfaces, upper surface 18 and lower surface 20. Substrate 10 alsoincludes side portions 22 on its elongated sides thereof.

Special consideration is now given to the unique lead frameconfiguration on substrate 10. The lead frame is about 0.005 inch thickin this embodiment and includes a plurality of spaced finger membersgenerally designated by the numeral 24. Each finger member 24 includesinner portions 26 which mutually converge on recess 12. Intermediatelead frame portions 28 radially extend from inner portions 26 anddiverge outwardly towards side portions 22 of substrate 10. It is animportant aspect of this invention that lead frame fingers 24 includeouter portions 30 which extend around substrate side portions 22. Outerportions 30 also continue around substrate side portions 22 and extendinwardly on the lower surface 20 toward the middle or central portionsthereof. Outer portions 30 then project arcuately from lower surface 20to form open ended ring-like resilient spring contacts 32. Springcontacts 32 project from lower surface 20 and are biased with respect tolower surface 20. That is, spring contacts 32 can yield in the directionof lower surface 20 upon application of a force in that direction. Itshould be noted that lead frame fingers 24 thus provide a continuousinterfaceless electrically conductive path for connecting asemiconductor device to conductors on a printed circuit board. In suchmanner, multiple points of interconnection are eliminated therebyincreasing the reliability in the electrical connection.

Special consideration should be given to the choice of the lead framematerial. The material should be chosen so that it has sufficientresiliency to form the integral spring contacts 32. Preferably, thematerial chosen for the lead frame should have a yield strength of atleast about 80,000 psi. Materials not having this characteristic may betoo weak to provide sufficient resiliency for spring contacts 32. Thepreferred material is copper alloy 638, as designated by the CopperDevelopment Association. Copper alloy 638 is an alloy containingapproximately 95 percent copper, 2.8 percent aluminum, 1.8 percentsilicon, and 0.4 percent cobalt. Copper alloy 638 is distributed underthe trade name CORONZE by Olin Corportion -Brass Division. Alloy 638 ispreferred not only because of its excellent mechanical properties, butbecause of its low cost, high electrical conductivity, and extremelyhigh oxidation and corrosion resistance. Further, alloy 638 forms anextremely important aluminum oxide film on its surface. This film istighly adherent to the underlying metal surface and is known to wet andbond well to many types of sealing glasses. The importance of thislatter property will become clear later in this description.

The discrete lead frame finger members 24 can be temporarily connectedtogether by a frame prior to attachment to the substrate 10 as is oftendone in the semiconductor packaging art. After stamping and forming, thelead frame then can be attached to the substrate 10 as shown mostclearly in FIG. 1. Preferably, the lead frame fingers 24 are attached tothe upper surface 18 by a layer 34 of vitreous sealing glass such as7047 distributed by Corning Glass, Inc. Corning Glass 7047 fuses to andbonds with the tough aluminum oxide film of the alloy 638 substrate andlead frame, and closely matches its comparatively high coefficient ofthermal expansion. This combination yields a high-strength and durablehermetic seal. The lead frame can be attached by screening the glasslayer 34 onto upper surface 18, the bottom surface 20, and the sideportions 22; heating it to its softening point, placing the lead frameon the softened layer, and then cooling it to harden the glass.Preferably, glass layer 34 is approximately 0.005-0.010 inch thick. Theglass should remain in this state until the final package subassembly isfired.

A stud 36 serves as a thermally conductive heat sink member. Stud 36includes a rectangular head 38 with a flat upper surface 40 forreceiving a semiconductor device. Stud 36 includes a rod-like bodyportion 42 depending from the head 38. A rectangular key extension 44bridges the head 38 and rod portion 42 of stud 36 and provides automaticalignment of the spring contacts 32 with corresponding printed circuitboard conductors as will be discussed hereinafter. Stud 36 is preferablyof a heat conductive metal. Berylliumcopper alloy 172, 1/2 hard, hasproven to provide extremely satisfactory results. The underside of head38 is sealed to surface 14 of recess 12 by glass layer 46. Preferably,this is accomplished using a vitreous glass preform.

A semiconductor device 48, such as an integrated circuit chip, is bondedto surface 40 of stud 36. Preferably, this is accomplished by an epoxyadhesive layer 50, such as EPO-TEK H-61, distributed by Epoxy TechnologyIncorporated of Watertown Massachusetts. Semiconductor device 48 can bebonded directly to surface 40 if electrical isolation is not desired.Furthermore, other methods of attaching the semiconductor device 48 tostud 36 can be used. For example, a beryllium oxide disc can be attachedto surface 40 by a gold preform and device 48 attached to the uppersurface of the disc by a silicon-gold eutectic bond. Various regions onthe semiconductor device 48 are attached to the inner portions 26 oflead frame finger members 24 by filamentary wires 52. This can beaccomplished by known ultrasonic or thermocompression bonding. It shouldalso be noted that one or more filamentary wires can be used to connecta portion of semiconductor device 48 to stud 36. The stud 36 can beused, if desired, as an electrically conductive ground contact as willbecome clear later in this description.

A cover member 54 has generally the same peripheral dimensions assubstrate 10. However, cover member 54 has a rectangular opening 56which is slightly larger than the recess 12 in the substrate 10. Covermember 54 is preferably of copper alloy 638 as is substrate 10 and thelead frame. Cover member 54 is mounted coextensive with the intermediateportions 28 of lead frame finger members 24. Preferably, this isaccomplished with a vitreous glass layer 58 which has been previouslyscreened and fired on the bottom surface of cover member 54. The covermember 54 is placed on the upper surface of substrate 10 with glasslayer 58 touching the intermediate lead frame portions 28. It should benoted that opening 56 prevents any covering of the inner portions 26which would hamper the filamentary wire bonding to device 48. Thesubassembly is then heated to reflow glass layers 34 and 58 topermanently secure the lead frame. Glass layer 46 which seals stud head38 is also reflowed at the same time.

By providing a rigid cover coextensive with the intermediate lead frameportions 28, the majority of the lead frame is thus sandwiched betweentwo rigid surfaces thereby providing a maximum strength packageconstruction. A lid 62, such as alloy 638, is sealed to the cover 54using a solder ring 60 to provide a finished hermetically sealedpackage.

It should be understood that the substrate 10 with the unique lead frameconfiguration as embodied in this invention provides a subassembly whichcan be packaged in a variety of manners. Consequently, it can be sold asa separate unit. Similarly, the substrate 10, stud 36, lead frame andcover member 54 form a subassembly which can be shipped to asemiconductor device manufacturer to complete the package. Suchmanufacturer would mount the semiconductor device 48 to the stud 36 andbond the various regions of the device to the exposed inner portions 26of the lead frame. Then the lid 62 can be sealed to the cover member 54to form the completed package.

Attention is now turned a distinctive design for mounting the packageonto a printed circuit board 64. The printed circuit board 64 is of aunique construction which provides excellent heat dissipation for thesemiconductor device 48 and further provides internal means for makingelectrical connection to a specified region of the device 48. Circuitboard 64 is of a multi-layered construction having an upper layer 66, amiddle layer 68 and a lower layer 70. Upper layer 66 and lower layer 70are made of fully cured or A-stage copper clad epoxy glass. Upper layer66 and lower layer 70 are bonded to middle layer 68 with an adhesive(not shown), such as semi-cured or B-stage epoxy glass. Middle layer 68is a thermally and electrically conductive metal, such as aluminum orcopper. In this example, upper layer 66 is approximately 0.010 inchesthick, middle layer 68 is approximately 0.042 inches thick, and lowerlayer 70 is approximately 0.010 inches thick. Circuit board 64 has acentrally located apertures 72 therein. A plurality of printed circuitmetallic conductors 74 are located on the external surfaces of layers 66and 70. Conductors 74 on opposite sides of the printed circuit board areinterconnected with plated-through via holes (not shown). Conductors 74correspond with the spring contacts 32 of the lead frame.

A receptacle 76 is preferably provided to receive rod portion 42 of stud36. Receptacle 76 is generally of an annular shape and is formed of aheat conductive spring metal, such as beryllium-copper alloy CA 172. Theupper surface of the receptacle 76 is countersunk and includes anindentation 78 in its periphery which corresponds with key 44 of stud36. A radially extending projection 80 around the receptacle 76 providesan abutment for engaging the printed circuit board as shown most clearlyin FIG. 2. Receptacle 76 includes a bore 82 extending longitudinallyabout its major axis. The bore 82 is slightly larger than the diameterof rod portion 42 of stud 36. In this example, it is nominally 0.065inches in diameter, with rod portion 42 being nominally 0.062 inches indiameter. In such manner, rod portion 42 can be slidably mounted withinreceptacle 76. The lower portion of receptacle 76 includes two slitsrunning longitudinally approximately half way up the length of thereceptacle. Two tangs 84 and 86 are formed integral with receptacle 76so as to exert a constant outward spring bias force against the ferrule88. Each tang is mutually connected by the upper portion of thereceptacle. The receptacle 76 is permanently soldered or press fit intoopening 72 in circuit board 64 as shown in FIG. 2 with the side walls ofthe receptacle contacting the metallic middle layer 68. The package canbe readily mounted onto the printed circuit board 64 merely by insertingrod portion 42 into the receptacle 76. The key 44 is received byindentation 78 to automatically align the spring contacts 32 of thepackage with their corresponding conductors 74 on the printed circuitboard.

A ferrule 88 of thermally deformable metal surronds the lower portion ofreceptacle 76. Thermally deformable metals are those metals capable ofcontracting and expanding when subjected to a temperature change. Thesematerials exhibit two different crystallographic orientations, one inits martensite or low temperature phase, and the other in its austeniteor high temperature phase. The change in crystallographic orientationresults in a substantial amount of physical deformation between its twostates. This deformation is fully recoverable and can be repeatedindefinitely. A more detailed description of this material may be had byreference to U.S. Pat. No. 3,174,851 Beuhler et al. One such metalhaving thermally deformable characteristics is a nickel-titanium alloydistributed under the trade name Cryocon by Raychem Corporation. Oneembodiment of this material expands when it is cooled to below -30°C, atemperature below which the circuit board assembly will not be subjectedin normal use. By cooling the ferrule 88 with an aerosol spray can ofFreon, it will expand to such a degree that rod portion 42 of stud 36can be easily inserted into the receptacle 76. When the ferruletemperature returns to about -30°C or above, it will contract and clampthe tangs 84 and 86 against the package stud 42 with substantial force.In such manner, the package is securely held in place by the forcesupplied by the ferrule 88 over the range of temperatures to which thecircuit board foreseeably will be subjected. In order to remove thepackage one only need spray the ferrule with a cooling media such asFreon to expand it. Then the package can be easily lifted out of thereceptacle 76.

The assembly shown in FIG. 2 provides many advantages over the prior artsemiconductor packaging techniques. The unique lead frame configurationprovides a continuous interfaceless path between the semiconductordevice 48 and the printed circuit board conductors 74. Accordingly, thepossibility of faulty interconnections are substantially reduced. Ifdesired, each of the spring contacts 32 can include a barb to piercenon-noble metal plated circuit board conductors in order to accomplishelectrical connection per U.S. Pat. No. 3,853,382, Lazar et al.Preferably, however, each of the contacts 32 would include a suitablycoined and finished noble metal surface to contact the printed circuitboard conductors without piercing them. The package design provides easyconnectability with the printed circuit board without the necessity ofan expensive intermediate connector. Furthermore, no soldering isreqiured. Therefore, the time, expense and risks of the solderingoperation is eliminated. The package can be readily removed merelythrough the use of a cooling medium to expand ferrule 88 as hereinbeforedescribed. The unique assembly as just described provides excellent heatdissipation for the semiconductor device 48. The heat generated indevice 48 is conducted through stud 36 through receptacle 76 and intothe metallic middle layer 68 of the printed circuit board. The rodportion 42 extending underneath the printed circuit board also aids inheat dissipation. Another advantage of the metal core circuit board isthat the metallic middle layer 68 can be used as an electricallyconductive plane. For example, the device 48 can be connected to stud 36with one or more filamentary wires 52 in order to connect it with themetallic middle layer 68 through receptacle 76. The metallic middlelayer 68 can readily be connected to ground. This is convenient where aplurality of packages are attached to one printed circuit board. In suchmanner, the metallic middle layer 68 prvides a common low-inductanceground plane for all of the devices.

The unique lead frame structure of the present invention can be used ina package which does not utilize stud 36 to mount it on a printedcircuit board. FIGS. 3 and 4 show such an embodiment. Substrate 90includes a pedestal 92 adapted to receive semiconductor device 94. Anepoxy preform 96 can be used to mount the chip 94 onto pedestal 92. Leadframe 100 is bonded to substrate 90 through the use of glass layers 98on the substrate surfaces. A cover 102 encapsulates the completedpackage via a glass layer 102 after portions of the chip 94 have beenbonded to inner portions of the lead frame 100, for example, as byfilamentary wire interconnections 106. The package shown in FIGS. 3 and4 can be mounted onto the printed circuit board with the use of aclamping system which presses down on the top of cover 102 to urge thespring contact portions of the lead frame 100 against the correspondingconductors on the printed circuit board. One such clamping system isdisclosed in U.S. Pat. No. 3,942,854, entitled "Hold Down Device For UseIn Systems Employing Integrated Circuits," filed Oct. 9, 1974, by Kleinet al and having the same assignee as the present invention. However,the unique lead frame structure of the present invention eliminates theneed for the intermediate connector disclosed in that patent.

While a metallic substrate is preferred because of its excellent thermalconductivity and because of the thermal expansion coefficient matchingwith the lead frame material, a ceramic substrate can also be used ifdesired. Similarly, a variety of covers can be employed to meet themanufacturers needs and processing specifications. Therefore, while thisinvention has been described in connection with one specific examplethereof, no limitation is intended thereby except as defined in theappended claims.

I claim:
 1. A semiconductor chip package subassembly comprising:asubstrate having a first major surface, side portions, and a secondsurface opposite the first surface, said substrate having means thereonfor supporting a semiconductor chip; and a conductive lead frame havinga plurality of mutually spaced finger members, each finger member havingintegral inner, intermediate, and outer portions; at least the innerportions of the lead frame being bonded to the first surface of thesubstrate and converging on said supporting means, said intermediateportions being on said substrate first surface and extending from saidinner portions to the side portions of the substrate, said outerportions extending around said substrate side portions and formingspring contacts projecting from the second surface of the substrate,whereby the lead frame finger members provide a continuous interfacelesselectrically conductive path for connecting a semiconductor chipdirectly to conductors on a printed circuit board without the need of anintermediate connector or soldering.
 2. The subassembly of claim 1wherein the lead frame is a metal having a yield strength of at least80,000 psi.
 3. The subassembly of claim 1 wherein the substrate ismetallic and the lead frame is bonded to the substrate with a layer ofglass.
 4. The subassembly of claim 3 wherein the substrate and the leadframe are made of a copper alloy containing aluminum which forms anoxide film that facilitates bonding to said glass layer.
 5. Thesubassembly of claim 1 wherein said supporting means comprises a recessin the substrate first surface, said recess having an opening extendingto said second surface, and a metallic heat sink member mounted in therecess for attaching a semiconductor device thereto.
 6. The subassemblyof claim 1 which further comprises means for encapsulating asemiconductor device after it has been attached to the supporting meanson the substrate.
 7. The subassembly of claim 5 wherein the heat sinkmember is in the form of a stud with body portions extending from saidsecond substrate surface through said opening.
 8. The subassembly ofclaim 7 which further comprises means cooperating with said stud bodyportion for securing it in an aperture in a printed circuit board tothereby engage said spring contacts of the lead frame finger memberswith corresponding conductors on the circuit board.
 9. A package for asemiconductor chip which is readily connectable with a printed circuitboard without requiring intermediate connectors or soldering, saidpackage comprising:a substrate having an upper major surface, sideportions, and a lower surface, said substrate having an opening thereinextending orthogonally to the major surface; a thermally conductive studmounted in the substrate opening, said stud having an upper end face forreceiving a semiconductor chip and lower body portions extending fromthe substrate lower surface; a conductive lead frame having a pluralityof mutually spaced finger members, each finger member having integralinner, intermediate, and outer portions; at least the inner portions ofthe lead frame being bonded to said substrate upper surface andconverging on said opening, said intermediate portions being on saidsubstrate upper surface and extending from said inner portions to theside portions of the substrate, said outer portions extending aroundsaid substrate side portions and forming spring contacts projecting fromthe lower surface of the substrate; and a rigid cover member on thesubstrate upper surface and sandwiching the intermediate portions of thelead frame therebetween for encapsulating a semiconductor chip bonded tosaid stud end face; wherein the lead frame of the package provides acontinuous interfaceless electrically conductive path for connecting thechip directly to conductors on the printed circuit board without theneed of an intermediate connector or soldering.
 10. The package of claim9 which further comprises means cooperating with said stud body portionsfor securing it in an aperture in a printed circuit board to therebyengage said spring contacts of the lead frame finger members withcorresponding conductors on a printed circuit board.
 11. The package ofclaim 7 wherein the substrate is metallic, and the lead frame is bondedto the substrate with a layer of glass.
 12. The package of claim 5wherein the substrate and the lead frame are made of a copper alloycontaining aluminum which forms an oxide film that facilitates bondingto said glass layer.